New Trends in Asymmetric Phase Transfer Catalysis

Abstract

Dedicated to Prof. Franco Cozzi on the occasion of his birthday.Phase Transfer Catalysis (PTC) is a powerful tool to perform reactions in a practical fashion, both in laboratory and industrial scale. Significant cost savings and major process improvements can be achieved in reactions performed under PTC conditions. In the last few years remarkable results in stereoselective reactions were achieved using chiral, non-racemic quaternary ammonium salts. Moreover, the use of bulky, chiral phosphate anions paired with achiral cations to generate lipophilic ion pairs allowed to design new avenues for the stereoselective construction of important building blocks. Hydrogen bond interactions were also shown to provide new pathways for asymmetric nucleophilic substitutions using insoluble reagents under PTC conditions. This Review will focus on recent advances in developing practical synthetic routes to construct molecules in a stereoselective fashion under PTC conditions.

Chiral Quaternary Ammonium Salt‐Catalyzed Enantioselective Addition Reactions of Hydantoins

“…We started our investigations by optimizing the addition of the alanine-based hydantoin 1a to acrylate 3a (Table 1) using the chiral ammonium salts AÀ E (Figure 1). [32][33][34][35][36][37][38][39][40][41] First experiments using Maruoka's catalyst A [37] and our bifunctional ammonium salt B [38] under classical phase-transfer conditions did not allow for any asymmetric induction (entries 1 and 2). Using the recently established spirobiindane-based ammonium salt C1 [39][40][41] resulted in low, but measurable, first levels of enantioselectivity (entry 3).…”

Enantioselective organocatalytic strategies to access noncanonical α-amino acids

“…Indeed, to obtain quaternary AAs under enantioselective PTC conditions, aldimines—but not ketimines—derived from α-alkyl glycine esters are generally used. 64,69 However, for the large-scale production of AAs, the use of a benzophenone ketimine and a t -butyl ester group is not optimal for cost reasons and poor atom economy. Aiming at the large-scale production of AAs, Maruoka and Ikunaka demonstrated that with the use of the binapthyl catalyst ent - 21 , a readily available benzaldimine derived from glycine ethyl ester could be employed (Scheme 24).…”

“…A plethora of structures, based on either Cinchona alkaloids or many other chiral scaffolds, capable of promoting this reaction with outstanding selectivity (ee > 90%) have appeared in the literature. 64 Many of these catalysts proved to be useful for unrelated PTC reactions as well. Arguably, Maruoka catalysts, such as 20 and its simplified version 21, stand out for their efficiency and generality ( Scheme 21 ).…”

State Of The Art Of Supported Phase Transfer‐Catalysts Onium Salt‐Based

“…A wide variety of catalysts have been developed during the last decades and the market for phase transfer catalysts has steadily increased over time reaching a size of 1,000 USD million [18] …”